Blow molded plastic containers are made by forming a parison which is subsequently inflated in a blow mold cavity having the configuration of the container. Parisons have been formed by extrusion and by injection molding. One technique for forming unoriented or low orientation bottles involves injection molding the parison in an injection mold having a core pin on which the parison is retained. The core pin and parison are transferred to a blow mold where pressurized gas (air) is used to inflate the hot parison. The core pin may be maintained at a temperature high enough to insure that the parison remains in a sufficiently soft state for easy inflation. This technique is not economically appropriate for blow molding highly molecularly oriented containers because the parison must be cooled to the orientation temperature range before the bottle can be blown. Another technique involves injection molding parisons in a mold having a cooled cavity and cooled core pin to rapidly solidify the parison. The parison is removed from the mold and stored. Later, the cold parison is reheated and inflated into conformity with a blow mold cavity. The latter process can produce highly molecularly oriented containers if the parison is reheated to an appropriate temperature within the orientation temperature range for the polymer prior to the blow molding step.
Molecular orientation is accomplished by stretching certain polymeric materials at a temperature within the orientation temperature range of the particular polymer. Stretching sheet or film along orthogonal axes produces biaxial orientation. Molecularly oriented materials have improved physical properties including superior impact resistance, increased resistance to creep, increased stiffness, and increased resistance to rupture when compared with the same material in an unoriented state.
Biaxial orientation of blow molded containers may be accomplished by blow molding a parison which is at a temperature within the orientation temperature range of the polymer. A high degree of orientation results in high resistance to creep, but can present problems such as reduced optical clarity, stress whitening, and cracking. The degree of orientation of the container can be measured by a standard test procedure (ASTM D-1504) which yields data in p.s.i. which are representative of the degree of orientation and are referred to as "Orientation Release Stress" (ORS).
For a given polymer and end use application, there is an optimum level of orientation as determined by orientation release stress (ORS), which may be below the maximum possible orientation level. For example, a property which deteriorates with attempts to achieve high levels of orientation is optical transparency. Many polymers crack, craze, stress whiten, show haze or otherwise become unsightly when highly oriented.
The amount of orientation in a container blow molded from a polymeric material is affected by the conditions under which the material is oriented. For example, in a bottle higher levels of circumferential and axial orientation result from increasing the amount of stretch in the circumferential and axial directions, by increasing the stretching rate, and by decreasing the stretching temperature.
For nitrile rubber containing acrylonitrile polymeric materials (such as those polymeric materials disclosed in U.S. Pat. No. 3,426,102 to Solak or U.S. Pat. No. 3,819,762 to Howe) an orientation release stress in the circumferential direction in excess of 500 p.s.i., preferably on the order of 650 p.s.i., provides adequate creep resistance for pressurized beverage bottles having a high contaner volume to weight ratio. See U.S. Pat. No. 3,786,221 to Silverman, and McChesney et al applications Ser. Nos. 319,380 now U.S. Pat. No. 3,934,743, 597,678 and Ser. No. 516,110 now U.S. Pat. No. 3,984,498, all assigned to the present assignee, hereby incorporated by reference. The improvement in physical properties due to orientatin also allows the wall thickness of the bottle to be reduced for a savings of polymeric material over that required for a non-oriented bottle.
While it is known to form molecularly oriented nitrile bottles by blow molding an injection molded parison, and such techniques have met with some success, generally they have not been economically practical for carbonated beverage bottles. The reason has been that if the bottle is oriented by stretching sufficiently to develop the properties required of containers for carbonated beverages (assuming a wall thickness thin enough to be economic), stress whitening, crazing or cracking has been observed to occur, making the container unattractive.
Further analysis of this phenomenon has brought the realization that cracking, crazing or stress whitening primarily develop at or near the inner surface portion of the bottle wall. This is due to the fact the the inside of the parison is stretched to a much higher extent, proportionally, than the outside. It has been found that the degree of orientation is not constant across the bottle wall thickness, but on the contrary varies substantially across the wall, and at or near the inner surface portion of the wall is sufficiently high to give rise to these problems.
In an effort to compensate for the difference in stretch between the inside and the outside, a method of heat treatment is disclosed in McChesney et al U.S. Pat. No. 3,934,743, for achievng a more uniform circumferential orientation across the thickness of the bottle sidewall. This is accomplished by imparting a radial temperature gradient to an axial zone of the sidewall of the parison prior to blow molding the parison into a bottle. The inner surface of the parison is made hotter than the outer surface of the parison to offset the difference in stretch.
Parisons suitable for blow molded, highly oriented containers are injection molded using a die cavity for the external configuration and a coaxial core pin for the internal configuration. Production efficiency requires that the molding cycle time be reduced to a minimum. The molding cycle time can be minimized if the cavity and core pin are cooled by circulating coolant to solidify the polymer rapidly. As the molten polymer is forced into a conventional cooled cavity and cooled pin injection mold, the polymer contacts the relatively cool cavity and core pin surfaces with a resulting increase in polymer viscosity at and near these mold surfaces and the formation of a solidifying layer on the mold surfaces. The continued addition of molten polymer to complete filling of the mold causes stresses to be developed in this solidifying layer at and near surfaces of the parison, which stresses are retained in the finished parison. Polymers having high viscosity or elasticity or both in the melt processing temperature range are susceptable to high residual stresses in the molded article. The later deliberately induced molecular orientation stresses add to these mold induced residual stresses. The higher degree of stretching of the inner surface of the container during blow molding coupled with the injection mold induced residual stresses, particularly those stresses at or near the inner surface of the parison, leads to excessive stress which may manifest as stress whitening, crazing or cracks in the container. These defects are undesirable because larger cracks may lead to failure by bursting of the container and smaller cracks impair the optical quality of the container by causing hazy or other poor appearance areas in the container wall. As a consequence, a container can not be oriented to the extent at which these defects appear. A higher degree of orientation could be achieved if these defects could be avoided.
Parisons reheated from ambient temperature require long periods of time to achieve temperatures appropriate for blow molding to high degrees of molecular orientation, involve handling, storage and cleanliness problems, and require the expenditure of additional energy. Further, the parison must be thoroughly cooled in the mold to be self-supporting when removed, thereby requiring relatively long injection mold cycle times. Of course, the parison, hot from the mold, can be quenched in water, but that causes distortion or surface degradation and requires drying.
Parisons blown directly from the core pin on which they were molded are not suited to production of highly oriented bottles in economic cycle times because the time required to temperature condition the parison to the lower temperature required for high orientation is great.